Magnetic resonance imaging apparatus

ABSTRACT

In a magnetic resonance imaging apparatus according to an embodiment, a transmitting coil applies a radio-frequency magnetic field to a subject placed in a static magnetic field. A receiving coil receives a magnetic resonance signal emitted from the subject owing to an application of the radio-frequency magnetic field. A balun is connected to the receiving coil, and suppresses an unbalanced current induced in the receiving coil. An overheat protection circuit indicates that the balun is abnormal when a temperature of the balun exceeds a temperature threshold. An imaging control unit stops imaging when the overheat protection circuit indicates an abnormality of the balun.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of copending application Ser. No. 12/854,414 filedAug. 11, 2010, which claims priority based on Japanese PatentApplication No. 2009-187353 filed Aug. 12, 2009, and Japanese PatentApplication No. 2010-176443 filed Aug. 5, 2010, the entire contents ofeach of which are hereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments described herein relate generally to a magnetic resonanceimaging apparatus.

BACKGROUND

A magnetic resonance imaging apparatus is an apparatus that images aninside of a subject by using a magnetic resonance phenomenon. Suchmagnetic resonance imaging apparatus includes a static magnetic-fieldmagnet that generates a static magnetic field in a scan zone, atransmitting coil that applies a radio-frequency magnetic field to asubject placed in the static magnetic field, and a receiving coil thatreceives a magnetic resonance signal emitted from the subject owing toan application of a radio-frequency magnetic field.

In a magnetic resonance imaging apparatus, the receiving coil isgenerally arranged on the inner side of the transmitting coil. For thisreason, when a radio-frequency magnetic field and an electric field areapplied to a subject by the transmitting coil, the radio-frequencymagnetic field and the electric field are also applied to the receivingcoil. As a result, a current and a voltage are induced in the receivingcoil. In this way, when a current and a voltage are induced in thereceiving coil, a loss of electric power supplied by the transmittingcoil may be produced; a spatial distribution of the radio-frequencymagnetic field may become non-uniform; the receiving coil may generateheat; and/or a part of the receiving coil may be broken, in some cases.

Therefore, aiming to suppress a current and a voltage induced in areceiving coil, a magnetic resonance imaging apparatus sometimesincludes a BALanced UNbalanced (BALUN) in some cases (for example, seeJP-A H7-136145).

However, the conventional technology described above has a possibilitythat when a strong current and a strong voltage beyond an assumption areinduced in the receiving coil, the safety of imaging may decreases, asexplained below.

Specifically, when a strong current and a strong voltage beyond anassumption are induced in the receiving coil owing to an application ofa radio-frequency magnetic field and an electric field by thetransmitting coil, the BALUN sometimes generates heat with the currentand the voltage in some cases. When the BALUN overheats, there is apossibility that the BALUN may be broken, or a subject may suffer a burndue to heat generation of the BALUN. This is not limited to BALUNs, andsimilarly occurs in other electronic circuits connected to a receivingcoil, for example, a trap circuit that switches between drive and stopof the receiving coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a general configuration of a MagneticResonance Imaging (MRI) apparatus according to an exemplary embodiment;

FIG. 2 is a schematic diagram for explaining details of a receiving coilaccording to the exemplary embodiment;

FIG. 3 is a functional block diagram of a configuration of a coilcircuit unit according to the exemplary embodiment;

FIG. 4 is a schematic diagram that depicts details of a BALancedUNbalanced (BUM) and an overheat protection circuit according to theexemplary embodiment;

FIG. 5 is a schematic diagram for explaining operation of the overheatprotection circuit according to the exemplary embodiment;

FIG. 6 is a functional block diagram of a detailed configuration of areceiving unit, a sequence control unit, and a control unit according tothe exemplary embodiment;

FIG. 7 is a flowchart of a process procedure of imaging control by theMRI apparatus according to the exemplary embodiment;

FIGS. 8-11 are schematic diagrams for explaining modifications of theoverheat protection circuit according to the exemplary embodiment;

FIG. 12 is a schematic diagram for explaining a case where an overheatprotection circuit is arranged in a vicinity of a trap circuit accordingto the exemplary embodiment;

FIG. 13 is a cross-sectional view that depicts an example of a compositecable according to the exemplary embodiment;

FIG. 14 is a schematic diagram of a structure of a toroidal BALUNaccording to the exemplary embodiment;

FIG. 15 is a circuit diagram of the toroidal BALUN according to theexemplary embodiment;

FIG. 16 is a schematic diagram of a general configuration when anoverheat protection circuit is applied to the toroidal BALUN accordingto the exemplary embodiment; and

FIG. 17 is a schematic diagram of a configuration of the toroidal BALUNshown in FIG. 16.

DETAILED DESCRIPTION

A magnetic resonance imaging apparatus according to an embodimentincludes a transmitting coil, a receiving coil, a balanced to unbalancedtransformer (BALUN, an overheat protection circuit, and an imagingcontrol unit. The transmitting coil applies a radio-frequency magneticfield to a subject placed in a static magnetic field. The receiving coilreceives a magnetic resonance signal emitted from the subject owing toan application of the radio-frequency magnetic field. The BALUN isconnected to the receiving coil, and suppresses an unbalanced currentinduced in the receiving coil. The overheat protection circuit indicatesthat the BALUN is abnormal when a temperature of the BALUN exceeds atemperature threshold. The imaging control unit stops imaging when theoverheat protection circuit indicates an abnormality of the BALUN.

The magnetic resonance imaging apparatus according to the embodimentwill be explained below in detail with reference to the accompanyingdrawings. Hereinafter, the magnetic resonance imaging apparatus isreferred to as the MRI apparatus.

First of all, a general configuration of the MRI apparatus according tothe embodiment is explained below. FIG. 1 is a schematic diagram forexplaining a configuration of an MRI apparatus 100 according to theembodiment. As shown in FIG. 1, the MRI apparatus 100 includes a staticmagnetic-field magnet 1, a gradient coil 2, a gradient magnetic-fieldpower source 3, a couchtop 4, a couch control unit 5, a transmittingcoil 6, a transmitting unit 7, a receiving coil 6, a receiving unit 9, asequence control unit 10 and a computer system 20.

The static magnetic-field magnet 1 is a magnet formed in a hollow drumshape, and generates a uniform static magnetic field in a space in itsinside. As the static magnetic-field magnet 1, for example, a permanentmagnet or a super conducting magnet is used.

The gradient coil 2 is a coil formed in a hollow drum shape, and isarranged on the inner side of the static magnetic-field magnet 1. Thegradient coil 2 is formed of three coils in combination corresponding tox, y, and z axes orthogonal to one another, and the three coils generaterespective gradient magnetic fields of which magnetic field strengthsvary along the x, y and z axes, respectively, by individually receivinga current supply from the gradient magnetic-field power source 3, whichwill be described later. It is assumed that the z axis direction is thesame direction as that of the static magnetic field. The gradientmagnetic-field power source 3 is a device that supplies a current to thegradient coil 2.

The gradient magnetic fields of the x, y, and z axes generated by thegradient coil 2 correspond to, for example, a slice-selective gradientmagnetic field Gs, a phase-encoding gradient magnetic field Ge, and areadout gradient magnetic field Gr, respectively. The slice-selectivegradient magnetic field Gs is used for arbitrarily setting a scan crosssection. The phase-encoding gradient magnetic field Ge is used forchanging the phase of a magnetic resonance signal in accordance with aspatial position. The readout gradient magnetic field Gr is used forchanging the frequency of a magnetic resonance signal in accordance witha spatial position.

The couchtop 4 includes a top plate 4 a on which a subject P is to beplaced. Under the control of the couch control unit 5, which will bedescribed later, the couchtop 4 inserts the top plate 4 a on which thesubject P is placed into a hole (a scanning space) of the gradient coil2. Usually, the couchtop 4 is placed such that the longitudinaldirection of the couchtop 4 is to be parallel to the central axis of thestatic magnetic-field magnet 1. The couch control unit 5 is a devicethat controls the couchtop 4 under the control of a control unit 26, andmoves the top plate 4 a in the longitudinal direction and upward anddownward by driving the couchtop 4.

The transmitting coil 6 is arranged on the inner side of the gradientcoil 2, and generates a radio-frequency magnetic field by receivingsupply of a radio-frequency pulse from the transmitting unit 7.

The transmitting unit 7 transmits a radio-frequency pulse correspondingto a Larmor frequency to the transmitting coil 6. Specifically, thetransmitting unit 7 includes an oscillating unit, a phase selectingunit, a frequency converting unit, an amplitude modulating unit and aradio-frequency power amplifying unit (RF-Amp), and the like. Theoscillating unit generates a radio-frequency signal of a resonancefrequency unique to a subject nucleus in the static magnetic field. Thephase selecting unit selects a phase of the radio-frequency signal. Thefrequency converting unit converts the frequency of a radio-frequencysignal output by the phase selecting unit. The amplitude modulating unitmodulates the amplitude of a radio-frequency signal output by thefrequency modulating unit in accordance with, for example, a sincfunction. The radio-frequency power amplifying unit amplifies aradio-frequency signal output by the amplitude modulating unit.

The receiving coil 8 is arranged on the inner side of the gradient coil2, and receives a magnetic resonance signal emitted from the subject Powing to an influence of the radio-frequency magnetic field describedabove. Upon receiving a magnetic resonance signal, the receiving coil 8outputs the magnetic resonance signal to the receiving unit 9.

The receiving unit 9 creates magnetic resonance data by inputting themagnetic resonance signal output by the receiving coil 8. Specifically,the receiving unit 9 includes a selector, a preamplifier, a phasedetector and an analog-digital converter. The selector selectivelyinputs a magnetic resonance signal output from the receiving coil 8. Thepreamplifier amplifies a magnetic resonance signal output from theselector. The phase detector detects a magnetic resonance signal outputfrom the preamplifier. The analog-digital converter creates magneticresonance data by converting a signal output from the phase detectorinto digital.

The sequence control unit 10 performs scanning of the subject P byactivating the gradient magnetic-field power source 3, the transmittingunit 7 and the receiving unit 9, based on sequence informationtransmitted from the computer system 20. When magnetic resonance signaldata is transmitted from the receiving unit 9 as a result of scanningthe subject P by activating the gradient magnetic-field power source 3,the transmitting unit 7 and the receiving unit 9; the sequence controlunit 10 transfers the magnetic resonance signal data to the computersystem 20.

The sequence information is information that defines a procedure forscanning, such as the strength of power to be supplied to the gradientcoil 2 by the gradient magnetic-field power source 3 and the timing ofsupplying the power, the strength of a radio-frequency signal to betransmitted to the transmitting coil 6 by the transmitting unit 7 andthe timing of transmitting the radio-frequency signal, and the timing ofdetecting a magnetic resonance signal by the receiving unit 9.

The computer system 20 performs total control of the MRI apparatus 100,data collection, image reconstruction and the like. The computer system20 particularly includes an interface unit 21, an image reconstructingunit 22, a storage unit 23, an input unit 24, a display unit 25 and thecontrol unit 26.

The interface unit 21 controls input and output of various signals thatare given and received to and from the sequence control unit 10. Forexample, the interface unit 21 transmits sequence information to thesequence control unit 10, and receives magnetic resonance signal datafrom the sequence control unit 10. When having received magneticresonance signal data, the interface unit 21 stores the receivedmagnetic resonance signal data into the storage unit 23 with respect toeach subject P.

The image reconstructing unit 22 creates spectrum data of a desirednuclear spin inside the subject P or image data, by performingpost-processing, i.e., reconstruction processing, such as Fouriertransform processing, on magnetic resonance signal data stored in thestorage unit 23.

The storage unit 23 stores magnetic resonance signal data received bythe interface unit 21, and image data created by the imagereconstructing unit 22, with respect to each subject P. The storage unit23 stores images taken by various imaging methods, such as the BlackBlood method, a delay contrast-enhanced imaging method and a taggingmethod.

The input unit 24 receives various instructions and information inputfrom an operator. As the input unit 24, a pointing device, such as amouse or a trackball, a selecting device, such as a mode switch, and aninput device, such as a keyboard, can be used as required.

The display unit 25 displays various information, such as spectrum dataor image data, under the control of the control unit 26. A displaydevice, such as a liquid crystal display, can be used as the displayunit 25.

The control unit 26 includes a Central Processing Unit (CPU) and amemory, both of which are not shown, and carries out total control ofthe MRI apparatus 100. Specifically, the control unit 26 controls a scanby creating sequence information based on imaging conditions input bythe operator via the input unit 24, and transmitting the createdsequence information to the sequence control unit 10; and controlsreconstruction of an image performed based on magnetic resonance signaldata sent from the sequence control unit 10 as a result of the scan.

Details of the receiving coil 8 are explained below. FIG. 2 is aschematic diagram for explaining details of the receiving coil 8.Although one unit of the receiving coil 8 is explained below as anexample, the MRI apparatus 100 includes a plurality of receiving coils8, such as a receiving coil for head, a receiving coil for abdomen and areceiving coil for spine. As shown in the figure, the receiving coil 8includes a coil loop unit 8 a and a coil circuit unit 8 b.

The coil loop unit 8 a detects a magnetic resonance signal emitted fromthe subject owing to an application of a radio-frequency magnetic fieldby the transmitting coil 6. The coil loop unit 8 a is connected to acoaxial cable 31 via the coil circuit unit 8 b. A plurality of BALUNs 32is inserted at predetermined points in the coaxial cable 31.

The coil circuit unit 8 b controls detection of a magnetic resonancesignal by the coil loop unit 8 a. The coil circuit unit 8 b is connectedto a received signal preamplifier 33 and a PIN-drive signal preamplifier34 via the coaxial cable 31. The coaxial cable 31 transmits a receivedsignal and a PIN drive signal in a superimposed manner. The cable usedin the embodiment is not limited to a coaxial cable, and can be a cablewhich includes a shield, for example, such as a composite cable shown inFIG. 13.

The received signal preamplifier 33 and the PIN-drive signalpreamplifier 34 are included in the receiving unit 9. Although not shownin FIG. 2, the received signal preamplifier 33 is connected to areceiving circuit that processes a received signal (magnetic resonancesignal) received by the receiving coil 8. The PIN-drive signalpreamplifier 34 is connected to a PIN drive circuit that controlsdriving of the receiving coil 8 by using a PIN drive signal.

The coil circuit unit 8 b is explained below in detail. FIG. 3 is afunctional block diagram of a configuration of the coil circuit unit 8b. As shown in the figure, specifically, the coil circuit unit 8 bincludes a trap circuit 41, a PIN diode 42, a tuned circuit 43, amatching circuit 44, a BALUN 45, and an overheat protection circuit 46.

The trap circuit 41 includes an active trap circuit and a passive trapcircuit, and operates the circuits, thereby protecting the receivingcircuit during a transmission of a radio-frequency magnetic field. ThePIN diode 42 activates the active trap circuit of the trap circuit 41when a PIN drive signal is passed from the PIN drive circuit. Thereceiving coil 8 is configured such that even when the active trapcircuit is not activated during the transmission of a radio-frequencymagnetic field, the passive trap circuit is activated, thereby beingcapable to protect the receiving circuit. The tuned circuit 43 matches aresonance frequency of the receiving coil 8 to a Larmor frequency. Thematching circuit 44 performs impedance matching between the receivingcoil 8 and the received signal preamplifier 33.

The BALUN 45 suppresses an unbalanced current induced to the receivingcoil 8. The overheat protection circuit 46 indicates that the BALUN 45is abnormal when the temperature of the BALUN 45 exceeds a temperaturethreshold. FIG. 4 is a schematic diagram that depicts details of theBALUN 45 and the overheat protection circuit 46. Although the BALUN 45and the overheat protection circuit 46 included in the coil circuit unit8 b are explained below as an example, each of the BALUNs 32 inserted inthe coaxial cable 31 is also provided with the overheat protectioncircuit 46.

As shown in FIG. 4, specifically, the BALUN 45 includes coils 45 a eachof which is formed by winding a coaxial cable, and a capacitor 45 b thatis connected in parallel with the coils 45 a. When unbalanced currentspass through the coils 45 a, magnetic fluxes generated with respectivecurrents passing through the GND side and the core-wire side cancel eachother out, consequently, the coils 45 a do not work as a coil; and whenan unbalanced current passes only through the GND side, the coil 45 aworks as a coil. Therefore, when an unbalanced current passes onlythrough the GND side, the coil 45 a and the capacitor 45 b operate as aparallel resonance circuit. When operating as a parallel resonancecircuit, the coil 45 a and the capacitor 45 b are adjusted to have ahigh impedance to match with the frequency of an induced voltagegenerated in the receiving coil 8 when transmitting a radio-frequencymagnetic field by the transmitting coil 6. Accordingly, an unbalancedcurrent passing through the GND side in the coaxial cable 31 can besuppressed.

On the other hand, the overheat protection circuit 46 includes atemperature fuse 46 a and a capacitor 46 b that are connected inparallel. The overheat protection circuit 46 is inserted on thecore-wire side in the coaxial cable 31, and arranged in the vicinity ofthe BALUN 45. When the temperature of heat generated by the BALUN 45exceeds a temperature threshold, the temperature fuse (an electricalsafety device, which interrupts electric current when heated to aspecific temperature, with a one-time fusible link) 46 a fuses, andturns to an open state. The capacitor 46 b is connected in parallel withthe temperature fuse 46 a.

Operation of the overheat protection circuit 46 is explained below. FIG.5 is a schematic diagram for explaining operation of the overheatprotection circuit 46. A received signal received by the receiving coil8 is a radio-frequency signal, and a PIN drive signal transmitted by thePIN drive circuit is a direct-current signal. For this reason, when thetemperature fuse 46 a is in continuity, as shown in FIG. 5, a receivedsignal passes through each of the temperature fuse 46 a and thecapacitor 46 b, and a PIN drive signal passes through only thetemperature fuse 46 a.

On the other hand, when the temperature fuse 46 a turns to an open statedue to overheat of the BALUN 45, a flow of a PIN drive signal is cutoff. As described above, even when a flow of a PIN drive signal is cutoff, the receiving circuit is protected by the passive trap circuitduring a transmission. Accordingly, when the temperature fuse 46 a turnsto an open state, only a received signal passes through via thecapacitor 46 b. In other words, when the temperature fuse 46 a turns toan open state from a continuous state due to overheat of the BALUN 45, aPIN drive signal does not pass through the receiving coil 8, while areceived signal passes through the receiving circuit.

Accordingly, even when the BALUN 45 is overheated and then thetemperature fuse 46 a turns to an open state, a magnetic resonancesignal received by the receiving coil 8 can be processed. In otherwords, even after the BALUN 45 is overheated and then the temperaturefuse 46 a turns to an open state, a scan can be continued.

Details of the receiving unit 9, the sequence control unit 10, and thecontrol unit 26 are explained below. FIG. 6 is a functional blockdiagram of a detailed configuration of the receiving unit 9, thesequence control unit 10, and the control unit 26. As shown in thefigure, the receiving unit 9 includes a receiving circuit 9 a and a PINdrive circuit 9 b, in addition to the above described function.

The receiving circuit 9 a processes a received signal (magneticresonance signal) received by the receiving coil 8. The PIN drivecircuit 9 b controls driving of the receiving coil 8 by using a PINdrive signal.

The sequence control unit 10 includes a first imaging-control unit 10 ain addition to the above described above function. The firstimaging-control unit 10 a executes a scan by driving the gradientmagnetic-field power source 3, the transmitting unit 7, and thereceiving unit 9, based on sequence information transmitted from thecomputer system 20.

After that, when an abnormality of a PIN drive signal is produced by theoverheat protection circuit 46, and when the receiving coil 8 with theproduced abnormality of the PIN drive signal is a specific receivingcoil, the first imaging-control unit 10 a immediately stops the scan.The specific receiving coil here is, for example, a receiving coil ofwhich temperature may rise in a short time. It is assumed that which ofthe receiving coils 8 is the specific receiving coil is preliminarilyset in the apparatus.

Moreover, when the first imaging-control unit 10 a stops a scan, thefirst imaging-control unit 10 a notifies the computer system 20 that thescan is stopped; and when an abnormality of a PIN drive signal isproduced, the first imaging-control unit 10 a notifies the computersystem 20 that the abnormality of the PIN drive signal is produced.

The control unit 26 of the computer system 20 includes a secondimaging-control unit 26 a in addition to the above described function.The second imaging-control unit 26 a creates sequence information basedon imaging conditions input from an operator via the input unit 24, andtransmits the created sequence information to the sequence control unit10, thereby executing a scam The second imaging-control unit 26 aexecutes a prescan for measuring the level of a received signal andother conditions, and a main scan for collecting imaging data,continuously in one time of an imaging protocol.

Moreover, while imaging, when it is notified from the sequence controlunit 10 that a scan is stopped, or that an abnormality of a PIN drivesignal is produced; the second imaging-control unit 26 a displays ontothe display unit 25 an error message indicating the notified contents.Furthermore, the second imaging-control unit 26 a stops a scan dependingon conditions.

A process procedure of the imaging control by the MRI apparatus 100according to the embodiment is explained below. FIG. 7 is a flowchart ofa process procedure of the imaging control by the MK apparatus 100according to the embodiment. The processing shown in the figuredescribes a series of processes related to a prescan and a main scanthat are to be performed in one time of an imaging protocol, and whenrepeatedly performing a plurality of imaging protocols, the series ofthe processes are to be repeatedly executed. A specific receiving coilto be a target according to which imaging is immediately stopped ishereinafter referred to as an “immediate-stop coil”.

According to the MRI apparatus 100 of the embodiment, as shown in FIG.7, to begin with, a prescan is started as the second imaging-controlunit 26 a of the computer system 20 transmits sequence information aboutthe prescan to the sequence control unit 10 (Step 501).

In the sequence control unit 10, when the first imaging-control unit 10a receives a PIN drive signal from the PIN drive circuit 9 b beforeperforming the prescan and while performing the prescan, the firstimaging-control unit 10 a determines whether the received PIN drivesignal is normal (Step S02).

if the PIN drive signal is normal (Yes at Step S02); the firstimaging-control unit 10 a starts a main scan by transmitting sequenceinformation about the main scan to the sequence control unit 10 (StepS04).

By contrast, if the PIN drive signal is abnormal (No at Step S02); thefirst imaging-control unit 10 a stops imaging without performing mainscan (Step S03).

Moreover, during the execution of the main scan, when the firstimaging-control unit 10 a receives a PIN drive signal from the PIN drivecircuit 9 b, the first imaging-control unit 10 a determines whether thereceived PIN drive signal is normal (Step S05).

If the PIN drive signal is normal (Yes at Step S05); the firstimaging-control unit 10 a continues the imaging, and finishes the mainscan (Step S06).

By contrast, if the PIN drive signal is abnormal (No at Step S05); thefirst imaging-control unit 10 a specifies a receiving coil with thedetected abnormality of the PIN drive signal (Step S07); and thendetermines whether the specified receiving coil is an immediate-stopcoil (Step S08).

If the specified receiving coil is an immediate-stop coil (Yes at StepS08); the first imaging-control unit 10 a immediately stops the imaging(Step S03).

By contrast, if the specified receiving coil is not immediate-stop coil(No at Step S08); the first imaging-control unit 10 a displays an errormessage onto the display unit 25 via the second imaging-control unit 26a (Step S09); and continues the imaging, and finishes the main scan(Step S06). In such case, because the PIN drive signal is abnormal, thesecond imaging-control unit 26 a causes the next imaging protocol not tobe executed.

In this way, according to the MRI apparatus 100, when an abnormality ofa PIN drive signal is produced by the overheat protection circuit 46,and when a receiving coil with the produced abnormality of the PIN drivesignal is an immediate-stop coil, the first imaging-control unit 10 aincluded in the sequence control unit 10 immediately stops imaging. Whena receiving coil with the produced abnormality of the PIN drive signalis not immediate-stop coil, the second imaging-control unit 26 aincluded in the computer system 20 stops imaging when an imagingprotocol in execution is completed.

As described above, the MRI apparatus 100 according to the embodimentincludes the transmitting coil 6 that applies a radio-frequency magneticfield onto a subject placed in a static magnetic field, the receivingcoil 8 that receives a magnetic resonance signal emitted from thesubject owing to the application of the radio-frequency magnetic fieldby the transmitting coil 6, and the BALUN 45 connected to the receivingcoil 8. Furthermore, the MRI apparatus 100 includes the overheatprotection circuit 46 that indicates that the BALUN 45 is abnormal whenthe temperature of the BALUN 45 exceeds a temperature threshold, and thefirst imaging-control unit 10 a and the second imaging-control unit 26 aeach of which stops imaging when an abnormality of a PIN drive signal isproduced by the overheat protection circuit 46. Therefore, according tothe embodiment, even when a strong current and a strong voltage beyondan assumption are induced to a receiving coil, the safety in imaging canbe improved by preventing overheat of an electronic circuit connected tothe receiving coil.

Although the embodiment is explained above in a case where when aspecified coil is an immediate-stop coil, imaging is immediately stoppedupon occurrence of an abnormality of a PIN drive signal, embodiments arenot limited to this. For example, it can be configured to determinewhether to stop imaging immediately, or to stop imaging when an imagingprotocol in execution is completed, depending on remaining imaging timeor type of imaging method.

In such case, when an abnormality of a PIN drive signal is produced bythe overheat protection circuit 46, and when imaging in execution isbeing performed by a specific imaging method, the first imaging-controlunit 10 a included in the sequence control unit 10 immediately stops theimaging. When imaging in execution is executed by an imaging methodother than a specific imaging method, or when an estimated temperatureat the termination of a protocol does not exceeds a temperaturethreshold, the second imaging-control unit 26 a included in the computersystem 20 stops the imaging when the imaging protocol in execution iscompleted. The “estimated temperature” here is calculated by, forexample, “radio-frequency output.times.remaining imagingtime.times.coefficient”. The “radio-frequency output” here is, forexample, a magnitude of electric power (W) output from the receivedsignal preamplifier 33, and a magnitude of electric power (W) suppliedto the transmitting coil 6.

Accordingly, for example, in a case of an imaging method by whichimaging can be re-executed after dealing with a point of abnormalityproduced due to overheat by replacing it, the imaging can be immediatelystopped. On the other hand, for example, in a case of imaging using acontrast agent, when an imaging time is short (an estimated temperatureat the imaging termination is lower than a threshold), which means animaging method unable to re-execute imaging immediately, the imaging canbe continued until the imaging protocol is completed.

The embodiment is explained below in a case where the overheatprotection circuit 46 includes the temperature fuse 46 a and thecapacitor 46 b that are connected in parallel. However, embodiments arenot limited to this. Modifications of the overheat protection circuitare explained below with reference to FIGS. 8-11.

First of all, for example, as shown in FIG. 8, a circuit in which thecapacitor 46 b is removed and only the temperature fuse 46 a is includedcan be used as an overheat protection circuit. In such case, when thetemperature fuse 46 a operates, not only a PIN drive signal, but also areceived signal is cut off, so that imaging is immediately stopped.

Alternatively, for example, as shown in FIG. 9, a circuit that includesa Positive Temperature Coefficient (PTC) element 46 c can be used as anoverheat protection circuit, instead of the temperature fuse 46 a. Asthe PTC element 46 c here, for example, a POLYSWITCH (registeredtrademark) can be used. Although the PTC element 46 c sometimes uses amagnetic material in some cases, the PTC element 46 c in a small sizewith little effect of the magnetic material has a large direct-currentresistance value. When the direct-current resistance value of the PTCelement 46 c is large, as the capacitor 46 b with a large capacity isconnected in parallel, so that a received signal passes through thecapacitor. Accordingly, a loss of a received signal can be reduced.Because a PIN drive signal relatively has an allowance with respect to acurrent and a voltage compared with a received signal, the PTC element46 c can be used even when its resistance value is high to a certainextent.

Moreover, although the embodiment is explained above in a case where theoverheat protection circuit 46 is inserted on the core-wire side in thecoaxial cable 31, the overheat protection circuit 46 can be inserted onthe GND (Shield) side. However, because an unbalanced current passesthrough on the GND side in the coaxial cable 31, when an overheatprotection circuit including a temperature fuse is provided, there is apossibility that an unbalanced current may exceed a rated value of thetemperature fuse.

For this reason, when an overheat protection circuit including atemperature fuse is connected on the GND side in the coaxial cable 31,for example, as shown in FIG. 10, a coil 46 d is serially connected tothe temperature fuse 46 a. Alternatively, as shown in FIG. 11, aparallel resonance circuit 46 e is connected to the temperature fuse 46a in parallel. Accordingly, an unbalanced current that is aradio-frequency passes through on the side of the capacitor 46 b, thepassage of an unbalanced current through the side of the temperaturefuse 46 a can be controlled. Because a PIN drive signal is a directcurrent, increase in the impedance can be ignored.

The embodiment is explained above in a case where the overheatprotection circuit 46 is arranged in the vicinity of the BALUN 45 inorder to detect overheat of the BALUN 45, embodiments are not limited tothis. For example, an overheat protection circuit can be arranged in thevicinity of a trap circuit instead of the BALUN 45.

FIG. 12 is a schematic diagram for explaining a case where an overheatprotection circuit is arranged in the vicinity of a trap circuit. Asshown in the figure, when an overheat protection circuit 50 a isarranged in the vicinity of a trap circuit 50, for example, the overheatprotection circuit 50 a is provided at an input end from which a PINdrive signal is input from the PIN drive circuit. The example shown inthe figure depicts a case of using an overheat protection circuit thatincludes only a temperature fuse.

When the overheat protection circuit 50 a is provided to the trapcircuit 50, it is configured such that the first imaging-control unit 10a of the sequence control unit 10 immediately stops imaging when anabnormality is detected by the overheat protection circuit 50 a.Accordingly, even when the trap circuit 50 is overheated as a strongvoltage beyond an assumption is induced in the receiving coil 8, it canprevent the trap circuit 50 from a failure, and a subject from sufferingburns due to heat generated by the trap circuit 50.

The technology according to the embodiment can be similarly applied toother than a BALUN and a trap circuit, for example, a cross diode usedfor overheat protection, and a choke coil used for the same purpose as aBALUN.

Recently, a phased array coil that includes a plurality of coil elementsis sometimes used as a receiving coil. The phased array coil often usesa composite cable. The composite cable is a cable that accommodates aplurality of coaxial cables and/or solid wires inside a shield.

FIG. 13 is a cross-sectional view that depicts an example of a compositecable. As shown in FIG. 13, a composite cable 60 includes a plurality ofcoaxial cables 61, a plurality of solid wires 62 and a shield 63. Theshield 63 is formed into a cylindrical shape and includes the coaxialcables 61 and the solid wires 62 in a tube. Although the composite cable60 that includes two of the coaxial cables 61 and four of the solidwires 62 is shown in FIG. 13 as an example, the number of the coaxialcables 61 and the number of the solid wires 62 to be accommodated in thecomposite cable 60 are not limited to these. Moreover, the compositecable 60 can accommodate only the coaxial cables 61, or only the solidwires 62.

When using such composite cable, a toroidal SALON is often used as acountermeasure against an unbalanced current passing through the cable.The toroidal BALUN is configured to control a magnetic field generatedby an unbalanced current, thereby controlling the unbalanced current.

FIG. 14 is a schematic diagram of a structure of a toroidal BALUN. FIG.15 is a circuit diagram of the toroidal BALUN. As shown in FIG. 14, forexample, a toroidal BALUN 70 includes a conductor 71 in a cylindricalshape that is formed hollow or by filling, such as Teflon (registeredtrademark), inside a peripheral wall. On a part of an outercircumference of the conductor 71, a slit 72 is formed in thecircumferential direction and a predetermined number of capacitors 73are provided so as to bridge the slit 72. The toroidal BALUN 70configured in this way is mounted so as to surround part of thecomposite cable 60 with the conductor 71. The shield 63 of the compositecable 60 and the conductor 71 are then soldered, thereby beingelectrically connoted to each other.

When an unbalanced current 81 passes through the shield 63 of thecomposite cable 60, a magnetic field (a magnetic flux 82 shown in FIG.14) is generated with the unbalanced current 81. The toroidal BALUN 70is adjusted so as to resonate with the frequency of the unbalancedcurrent 81. When the magnetic field is generated, a current passesthrough the conductor 71 of the toroidal BALUN 70 owing to mutualinduction, and then an inverse current is generated so as to cancel thecurrent. As a result, the magnetic field is canceled out, unbalancedcurrent does not pass through. In other words, as shown in FIG. 15,according to the toroidal BALUN 70, the conductor 71 works as aninductor and the magnetic flux 82 is blocked by a resonance circuitincluding the conductor 71 and the capacitor 73 so that unbalancedcurrent is controlled.

An overheat protection circuit can be applied to the toroidal BALUN 70configured in this way. In the toroidal BALUN 70, when unbalancedcurrent is suppressed, the capacitor 73 generates heat the most.Therefore, for example, as an overheat protection circuit is provided inthe vicinity of the capacitor 73, overheat of the capacitor 73, i.e.,overheat of the toroidal BALUN 70 is detected.

FIG. 16 is a schematic diagram of a configuration when an overheatprotection circuit is applied to the toroidal BALUN 70. As shown in FIG.16, for example, a phased array coil 90 includes a plurality of coilloop units 91 and a transmission-reception switching circuit 92. Thetransmission-reception switching circuit 92 is provided with, forexample, the BALUN 45 and the overheat protection circuit 46 shown inFIG. 3. The transmission-reception switching circuit 92 is connected toa preamplifier 93, transmission-reception switching drive circuits 94and a failure detecting circuit 95, via the composite cable 60.

The composite cable 60 includes, for example, the coaxial cables 61,solid wires 62 a and 62 b, and the shield 63. The coaxial cables 61transmit received signals (Radio Frequency (RF)) received by the coilloop units 91 to the preamplifier 93. The solid wire 62 a transmits aswitching signal for controlling switching of the transmission-receptionof the coil loop units 91 from the transmission-reception switchingdrive circuits 94 to the transmission-reception switching circuit 92.The shield 63 includes the coaxial cables 61 the solid wires 62 a and 62b in its inside. Although not shown in FIG. 16, the coaxial cable 61 andthe transmission-reception switching drive circuit 94 are provided toeach of the coil loop units 91. The number of coil loops can be one ormore. A predetermined number of the toroidal BALUNs 70 are mounted ontothe composite cable 60.

FIG. 17 is a schematic diagram of a configuration of the toroidal BALUN70 shown in FIG. 16. As shown in FIG. 17, the toroidal BALUN 70 includesa printed circuit board 74 that is farmed hollow or by filling, such asTeflon (registered trademark), inside a peripheral wall, and a copperfoil is formed as the conductor 71 over the surface of the printedcircuit board 74. On a part of an outer circumference of the conductor71, the slit 72 is formed in the circumferential direction (a planevertical to the cable) of the printed circuit board 74, and thecapacitors 73 are provided so as to bridge the slit 72. The conductor 71is then soldered to the shield 63 of the composite cable 60 on the innercircumference side of the printed circuit board 74, thereby beingelectrically connoted.

In the toroidal BALUN 70 configured in this way, when unbalanced currentis suppressed, the capacitor 73 generates heat the most. For thisreason, as shown in FIG. 17, a temperature fuse 96 is provided in thevicinity of the capacitor 73, as an overheat protection element. Thetemperature fuse 96 is wired with a twist pair cable, in order to avoidinfluence of a magnetic flux of the toroidal BALUN 70. One of theelectric wires protected by the shield 63 is connected to thetemperature fuse 96, for example, and used for overheat detection. Inthe example shown in FIG. 17, the solid wire 62 b included in thecomposite cable 60 is used as an electric wire for overheat detection.

Returning to FIG. 16, one end of the solid wire 62 b is connected to theoverheat protection circuit provided in the transmission-receptionswitching circuit 92, and the other end is connected to the failuredetecting circuit 95 via the temperature fuses 96 provided in the two ofthe toroidal BALUNs 70. Accordingly, as the failure detecting circuit 95detects a cutoff of the temperature fuse 96 by using the solid wire 62b, overheat of the capacitor 73 included in the toroidal BALUN 70 can bedetected.

Although it is explained above in a case where the electric wireconnected to the temperature fuse 96 is dedicated to detect overheat ofthe toroidal BALUN 70, the electric wire can be used also as an electricwire for transmitting a switching signal of the coils.

As described above, according to the embodiment, overheat protection canbe performed also against overheat of a toroidal BALUN.

While certain embodiments have been described, these embodiments havebeen presented by way of example only and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A magnetic resonance imaging apparatus, saidapparatus comprising: a transmitting coil configured to apply aradio-frequency magnetic field to a subject placed in a static magneticfield; a receiving coil, configured to receive a magnetic resonancesignal emitted from the subject in response to application of theradio-frequency magnetic field by the transmitting coil; an electroniccircuit configured to be connected to the receiving coil including adriving control circuit configured to control driving of the receivingcoil; and an overheat protection circuit configured to include atemperature fuse arranged between the receiving coil and said drivingcontrol circuit; wherein the temperature fuse is configured to become anopen circuit when a temperature of the electronic circuit exceeds atemperature threshold.
 2. The magnetic resonance imaging apparatusaccording to claim 1, wherein the electronic circuit comprises one of:(a) a balun, (b) a trap circuit, (c) a cross diode, and (d) a chokecoil.
 3. The magnetic resonance imaging apparatus according to claim 1,wherein the overheat protection circuit is configured to conduct amagnetic resonance signal received by the receiving coil after thetemperature fuse becomes an open circuit, through a receiving circuitconfigured to process the magnetic resonance signal.
 4. The magneticresonance imaging apparatus according to claim 2, wherein the overheatprotection circuit is configured to conduct a magnetic resonance signalreceived by the receiving coil after the temperature fuse becomes opencircuit, through a receiving circuit configured to process the magneticresonance signal.
 5. The magnetic resonance imaging apparatus accordingto claim 1, wherein the overheat protection circuit includes one of: (a)an inductor and (b) a parallel resonance circuit, serially connected tothe temperature fuse.
 6. The magnetic resonance imaging apparatusaccording to claim 2, wherein the overheat protection circuit includesone of: (a) an inductor and (b) a parallel resonance circuit, seriallyconnected to the temperature fuse.
 7. The magnetic resonance imagingapparatus according to claim 3, wherein the overheat protection circuitincludes one of: (a) an inductor and (b) a parallel resonance circuit,serially connected to the temperature fuse.
 8. The magnetic resonanceimaging apparatus according to claim 4, wherein the overheat protectioncircuit includes one of: (a) an inductor and (b) a parallel resonancecircuit, serially connected to the temperature fuse.
 9. The magneticresonance imaging apparatus according to claim 1, wherein the electroniccircuit comprises a balun including a resonance circuit configured toinclude a capacitor and an inductor.
 10. The magnetic resonance imagingapparatus according to claim 2, wherein the electronic circuit comprisesa balun including a resonance circuit configured to include a capacitorand an inductor.
 11. The magnetic resonance imaging apparatus accordingto claim 3, wherein the electronic circuit comprises a balun including aresonance circuit configured to include a capacitor and an inductor. 12.The magnetic resonance imaging apparatus according to claim 4, whereinthe electronic circuit comprises a balun including a resonance circuitconfigured to include a capacitor and an inductor.
 13. The magneticresonance imaging apparatus according to claim 9, wherein the inductorcomprises a cable configured to include a shield having two ends and tobe wound in a coil, and the capacitor is configured to connect both endsof the shield.
 14. The magnetic resonance imaging apparatus according toclaim 10, wherein the inductor comprises a cable configured to include ashield having two ends and to be wound in a coil, and the capacitor isconfigured to connect both ends of the shield.
 15. The magneticresonance imaging apparatus according to claim 11, wherein the inductorcomprises a cable configured to include a shield having two ends and tobe wound in a coil, and the capacitor is configured to connect both endsof the shield.
 16. The magnetic resonance imaging apparatus according toclaim 12, wherein the inductor comprises a cable configured to include ashield having two ends and to be wound in a coil, and the capacitor isconfigured to connect both ends of the shield.
 17. The magneticresonance imaging apparatus according to claim 9, wherein the inductoris arranged outside a cable configured to include a shield thatsurrounds at least part of the cable, and a slit is formed on anexternal conductor of which both ends are connected to the shield, andthe capacitor is connected to bridge the slit.
 18. The magneticresonance imaging apparatus according to claim 10, wherein the inductoris arranged outside a cable configured to include a shield thatsurrounds at least part of the cable, and a slit is formed on anexternal conductor of which both ends are connected to the shield, andthe capacitor is connected to bridge the slit.
 19. The magneticresonance imaging apparatus according to claim 11, wherein the inductoris arranged outside a cable configured to include a shield thatsurrounds at least part of the cable, and a slit is formed on anexternal conductor of which both ends are connected to the shield, andthe capacitor is connected to bridge the slit.
 20. The magneticresonance imaging apparatus according to claim 12, wherein the inductoris arranged outside a cable configured to include a shield thatsurrounds at least part of the cable, and a slit is formed on anexternal conductor of which both ends are connected to the shield, andthe capacitor is connected to bridge the slit.